Once the object-oriented version of EME is implemented (issue 224786) and sites have switched, we will remove support for v0.1b and delete the implementation. This bug will track that work as well as specific items to remove.
The first step will be to remove the flag to enable prefixed EME (issue 451117). At that time, we should also remove the ability to disable unprefixed and remove all the related strings for both.
Prefixed EME is now removed in master (M50), so I have closed this issue to reflect this and unblock other issues.
We will continue to land CLs to remove remaining prefixed-related code and implement refactorings that are now possible.